302 ' THE OCEAN WORLD. 



course. The neural surface is that upon which the majority of 

 molluscs move, and by which they are supported, and it is commonly 

 modified to subserve these purposes by the formation of a muscular 

 expansion or disk, called the foot. Three regions, in many genera 

 very distinctly divided from one another, may be distinguished in this 

 foot : an anterior, the Propodium (p p) ; a middle, the Mesopodium 

 (m s) ; and a posterior, the Metapodium (m t). In addition to these, 

 the upper part of the foot, or middle portion of the body, may be 

 prolonged into a muscular enlargement on each side, just below the 

 junction of the haemal with the neural region, the Epipodium (e p). 

 The mass of the body between the foot proper and the part of the 

 abdomen which bears the epipodium may be termed the mid-body, 

 or Mesosoma. On the upper part of the sides of the head are two 

 pairs of organs, namely, the eyes and tentacles. In the haemal region 

 the integument may be modified and raised up into a fold at the edges, 

 either in front or behind the anus. When so modified, it is called a 

 mantle, Pallium. In front of the anus again, the branchiae (f) project 

 as processes of the haemal region. Among the internal organs, the 

 heart (u v) lies in front of the branchiae in the haemal regions, the 

 nervous ganglia (x y z), of which there are three principal pairs, being 

 arranged around the alimentary canal, which they encircle. 



Such is the general type of the class Mollusca, of which, however, 

 the variations are innumerable. They are all soft-skinned animals, 

 without either articulated exterior or annular external skeleton. Their 

 nervous system, being without cerebro-spinal axis, is entirely composed 

 of ganglions, which are all reunited in the oesophagus without consti- 

 tuting in any case a lengthened median chain. Their digestive organs 

 are complete that is, they are provided with two apertures ; their 

 principal organs are symmetrical and according to a plan, usually 

 curving, by which their bodies are divided into two parts. 



The first series or subdivision, to which Milne Edwards has given 

 the name of Molluscoida, includes under that term the Bryozoaires, 

 Ascidians, and Tunicata. 



